Higher molecular weight hydrocarbon fractions used as feedstocks to prepare lubricating oil basestocks typically have an initial boiling point in 343° C.+ range. These feedstocks usually contain wax, irrespective of whether the fraction is derived from natural or synthetic sources. Most wax containing feedstocks are derived from naturally occurring sources, such as petroleum, bitumen and the like, but in the future more and more will be derived from synthetic crudes and hydrocarbon fractions produced by processes such as gas conversion, wherein natural gas or a gas comprising primarily methane is converted to a synthesis gas which, in turn, is used to synthesize hydrocarbons. The trend to lubricating oils having higher VI to meet government mandated standards leads to feedstocks having increasing wax contents. Basestocks meeting the Group I classification for motor oils are typically prepared using solvent techniques, while Groups II and III basestocks typically utilize catalytic techniques.
Feedstocks boiling in the range of from about 343 to about 566° C. or greater are used to prepare lubricating oils for motor vehicles, turbines, machining and the like. In order for a lubricating oil fraction to be useful as a lubricating oil base stock, the wax must be at least partially removed. This is accomplished by either solvent dewaxing or catalytic dewaxing. Most dewaxing facilities used to prepare Group I basestocks still employ solvent dewaxing, in which a chilled dewaxing solvent is slowly mixed with the lubricating oil fraction and the mixture slowly cooled, under conditions of agitation, down to the desired cloud or pour point temperature. Group II basestocks are typically prepared using either solvent or catalytic techniques. Group III and higher basestocks are prepared using catalytic techniques for dewaxing.
One method of dilution chilling dewaxing is the DILCHILLSM process (DILCHILLSM is a registered Service Mark of ExxonMobil Research and Engineering Company). DILCHILLSM is disclosed in U.S. Pat. No. 3,773,650. A number of improvements and modifications have been made to the basic concept of DILCHILL. For example it has been shown that in a vertically staged cooling tower, the velocity of the solvent at the injection points within each stage should be at least 5-30 times that of the peripheral velocity of the mixer blades. This results in greater filtration rates and higher dewaxed oil yields than could otherwise be obtained without the relatively high velocity solvent injection. It has also been shown that a combination of dilution chilling with scraped surface chilling is useful for dewaxing lubricating oils. Other methods teach adjusting the dewaxing solvent composition so that the waxy oil and solvent are immiscible near the last stage of the cooling zone. This results in superior dewaxed oil yields and higher filter rates when the waxy oil stock being fed to the tower is relatively high in viscosity and molecular weight. It is also known to partially predilute the waxy oil when the oil is a relatively heavy feed such as a resid or a bright stock before the oil is introduced into the chilling zone. However, in all of these DILCHILLSM dewaxing processes, it was thought that the rate of solvent addition to each stage should be adjusted so as to obtain the same or approximately equal temperature drops in each stage.
It is known that the DILCHILLSM process is improved when the waxy lube oil stocks are solvent dewaxed by contacting them with successive increments of cold dewaxing solvent at a plurality of points along the height of a vertical tower divided into a plurality of stages while agitating the oil solvent mixture in each stage to provide substantially instantaneous mixing of the waxy oil and solvent thereby precipitating wax from the oil. The well known shock chilling effect is avoided by adjusting the cold solvent addition to each stage in a manner so as to modify the temperature profile along the tower to ensure that the temperature drop per stage in the initial stages in which wax precipitation occurs is greater than the temperature drop per stage in the final or later stages in which wax precipitation occurs.
Various methods have been proposed to monitor wax crystallization. In one method, a laser beam reflected by wax crystals is used in determining the wax crystallization temperature of a hot dewaxing solvent upstream of solvent chillers. This is automatically achieved by an on-line method from a remote control point, in which a slipstream of solvent is passed through an attached solvent loop into a sample chamber in the loop, without being exposed to ambient conditions. As the sample is cooled, the beam reflections are detected and indicate the wax to crystallization temperature. Corrective measures can then be taken to prevent fouling of the chillers, if need be. Another method uses an electronic analyzer which involves the transmission of light or in the degree or intensity of reflected light associated with wax formation.
The above methods are directed to analyzing for total wax content. There is still a need to monitor the effectiveness of the wax crystallization process itself, especially as it relates to crystal size distributions.